Automatic high vacuum system



Sept. 27, 1966 T. J. HOLCE AUTOMATIC HIGH VACUUM SYSTEM 2 Sheets-Sheet 1 Original Filed Aug. 21, 1961 Thomas J.Ho]ce INVENTOR.

( TTORNEY Sept. 27, 1966 T. J. HOLCE AUTOMATIC HIGH VACUUM SYSTEM 2 Sheets-Sheet 2 Original Filed Aug. 21, 1961 Thomas J Holce INVENTOR ORNEY 2 #m mm QQX,

Vb NW Q United States Patent 3,275,221 AUTOMATIC HIGH VACUUM SYSTEM Thomas J. Holce, Forest Grove, 0reg., assignor, by mesne assignments, to Varian Associates, Palo Alto, Calif., a

corporation of California Continuation of application Ser- No. 132,614, Aug. 21,

1961. This application May 27, 1965, Ser. No. 459,231 Claims. (Cl. 230-2) This application is a continuation of my copending application, Ser. No. 132,614, filed Aug. 21, 1961 for improvements in automatic high vacuum system, which application will now be permitted to lapse without prejudice, in view of its being superseded by this present application.

This invention relates to high vacuum systems, and more particularly to an automatically cont-rolled high vacuum system.

It is a principal object of the present invention to provide an automatic high vacuum system in which automatic valving control affords optimum working pressures in the mechanical and diffusion pumps, protection against leaks and prevention of hot oil damage in the diffusion pump,

and optimum performance by the elimination of human operational errors.

Another important object of this invention is to provide an automatic high vacuum system which includes an improved vacuum sensing component assuring maximum precision and maintenance of optimum pressures.

A further important object of the present invention is the provision of an automatic high vacuum system which includes an improved high vacuum valve by which the automatic switching between the mechanical and diffusion pumps is rendered practicable.

A still further important object of this invention is the I provision of an automatic high vacuum system of simplified construction for economical manufacture ,and which affords long and faithful operation with a mini- 'mum of maintenance.

The foregoing and other objects and advantages of the present invention will appear from the following detailed description, taken in connection with the accompanying drawings in which:

FIG. 1 is a schematic diagram of an electric circuit of an automatic high vacuum system embodying features of this invention;

FIG. 2 is a foreshortened partially sectioned side eleva tion of a multiple high vacuum valve embodying features of the present invention; and

FIG. 3 is a partially sectioned bottom view of the valve,

as viewed from the right in FIG. 2.

The high vacuum system illustrated schematically in FIG. 1 is that of a substantially conventional vacuum evaporator, and is chosen merely as a form by which to describe the present invention. It includes an electric motor 10 for operating the mechanical vacuum pump 12 the inlet conduit 14 of which communicates selectively through the valves 16 and 18 with the bypass conduit 20 and the backing conduit 22, respectively. The valves 16 and 18 preferably are integrated into the unitary constrution illustrated in FIGS. 2 and 3 for mutual operation by the single electric motor 24. The backing conduit leads to the diffusion pump 26 the inlet 28 to which is releasably closed by the high vacuum flap valve 30 operated by the electric motor 32.

The bypass conduit and the diffusion pump inlet interconnect at the outlet 34 of the vacuum system which, in the embodiment illustrated is the entrance to a bell jar 36. The outlet 34 of the system effectively extends from the valves 16 and 30, and this outlet communicates through the conduit 38 with a vacuum sensing element 40 Patented Sept. 27, 1 966 bottom 50 and peripheral wall 52 forming the cylindrical vacuum chamber 54. The body is provided with a removable top plate 56 which is secured thereto by means of the elongated bolts 58. These bolts pass through registering openings adjacent the corners of the body and plate, and an intermediate length of the bolts projecting from the plate freely receive the spacer sleeves 60. Ap-

propriately positioned openings in the motor mounting bracket 62 receive the bolts therethrough for abutment of the bracket against the sleeves, and this assembly is secured firmly together by means of the nuts 64 mounted on the threaded end portions of the bolts.

A vacuum tight seal is provided between the valve body and top plate by means of the resilient O-ring 68 mounted in an annular groove in the underside of the top plate.

An inlet opening 70 is provided in the valve body for communication with the inlet conduit 14 to the mechanical pump. In the embodiment illustrated, the inlet conduit is secured to a mounting plate 72, by means of soldering,

brazing, or other means providing a vacuum tight seal and the mounting plate is provided with an opening 74 registering with the conduit. The mounting plate is removably secured to the valve body, as by means of the screws 76, and a vacuum tight seal between the plate and body is provided by the resilient O-ring 78 mounted in, an annular groove in the mounting plate surrounding the opening.

' A pair of diametrically opposed openings 80 and 82 are provided in the top plate 56 for communication with the valve body chamber 54, and these openings communicate with the valve body chamber 54, and these openings communicate with the bypass and backing conduits 20 and 22, respectively. These conduits are secured to the top plate by means of soldering, brazing, or other means which provides a vacuum tight seal.

Located within the valve body chamber 54 is a valve disc 84. A central opening therein receives the intermediate stepped section 86 of the drive rod 88, and these elements are secured firmly together for simultaneous rotation by means of the noncircular cross section illustrated, or by a press fit, soldering, or other means. The adjacent outer section of the drive rod is of smaller diameter and extends freely through an opening in the top plate, and a vacuum tight seal therebetween is provided by the resilient O-ring 90. The inner end of the drive rod is reduced in diameter and is received rotatably in the aligning socket 92 formed in the bottom 50 of the valve body. A thrust bearing 94 is mounted on this reduced end section of the drive rod between the valve body bottom and the shoulder 96. It is by means of this thrust bearing that the valve disc 84 is supported against axial displacement and thus is maintained closely adjacent the underside of the top plate 56.

Annular grooves surrounding the valve openings 80-, 82 in the top plate 56 support the resilient O-rings 98 which press against the adjacent surface of the valve disc 84. Thus, a vacuum tight seal is provided between the valve disc and the valve openings when closed by the disc. The valve disc is provided with a single opening 100 which is adapted, upon rotation of the disc, to'register with one or the other of the valve openings 80, 82. The arrangement of openings is such that the valve disc opening 100 registers selectively with the valve openings 80, 82 at intervals of rotation of the disc, and both valve openings are sealed by pressure contact of the disc modate a degree of misalignment of the shaft and rod without destroying the precise matching of the valve disc 84 with the adjacent surface of the top plate 56. In the embodiment illustrated, this flexible coupling is provided by the axially spaced coupling plates 104 and 106 secured releasably to the spaced, adjacent ends of the .shaft 102 and rod 88, respectively. Each plate is provided with a socket 108 and a diametrically opposed projecting pin 110, and the pin on one plate is received freely within the socket in the other plate.

The rigid support of the valve disc 84 by the thrust bearing 94, to prevent axial displacement of the disc away from the adjacent surface of the top plate 56, is

essential to the effective performance of the valve in vacuum systems, since the differential pressure on opposite sides of the disc is in the direction tending to force the disc away from the vacuum sealing rings 98.

Another improvement embodied in the present invention resides in the circuitry associated with the vacuum sensing Pirani transducer 40. Referring to FIG. 1 of the drawings, the Pirani resistance 120 and compensator resistance 122 are included with potentiometer 124 and fixed resistance 126 in a normal Wheatstone bridge arrangement. One end of the transformer winding 128 is connected to the junction 130 and the opposite end of the transformer winding is connected through the swamping resistance 132 to the opposite junction 134. The junction between the potentiometer 124 and fixed resistance 126 is connected to the input control grid of a linear A.C. amplifier 136, the output of which is connected through the rectifier 138 to an ammeter 140 calibrated in microns. The meter is bridged across the resistance 142 and potentiometer 144 for reference voltage, the potentiometer functioning as a meter adjustment.

The potentiometer 124 in the bridge circuit is connected to the input of an A.C. amplifier 150, and the output from the latter is connected to one end of the relay coil 152 which, as explained more fully hereinafter, controls the operation of the vacuum valves. Assuming the amplifier to be a single stage, or any odd number of stages, the opposite end of the relay coil is connected to the end of the second transformer winding 154 adjacent the first transformer Winding 128, and the opposite end of the second winding is connected to ground. Alternating current is supplied to the transformer windings from the primary winding 156.

In the operation of the vacuum sensing system described hereinbefore, alternating current is supplied to the Pirani element 120 through the swamping resistance 132 which functions, by its large resistance value, for example 21,000 ohms, to minimize the changes in current to the Pirani element as the resistance of the latter changes in response to changes in the degree of vacuum in the system. Resistance 126 is chosen to balance the junction to the amplifier 136 for zero volts to ground, and potentiometer 124 may be adjusted to balance its amplifier connection to ground for any given condition of the Pirani resistance.

Assuming now that the lower end of the second transformer winding 154 is at the positive portion of the alternating current supply, and assuming that the Pirani resistance 120 increases due to a decrease in pressure in the vacuum system, a positive going sine wave will be applied to the input control grid of the amplifier 150. Since the phase relationship between the transformer windings 128 and 154 supplying alternating current to the amplifier input and output has been properly chosen, the amplifier input and output will be in phase and the amplifier will conduct, thereby activating the control relay 152 for purposes explained more fully hereinafter.

Since the voltage applied to the input grid of the meter amplifier 136 is proportional to the change in resistance of the Pirani element 120, the linear output voltage from the amplifier may be rectified and applied to the meter to provide a direct indication of the degree of vacuum in the system.

The primary advantage derived from the vacuum sensing system described hereinbefore resides in the use of alternating current amplifiers and heat source for the Pirani transducer. By using alternating current amplifiers the inherent changes in transcondu-ctance of the amplifier tubes, as the tubes age or are exchanged, does not affect the firing point of the amplifiers, thus providing precise operation of the control relay 152.

The electrical circuitry illustrated in FIG. 1 functions to control automatically the valving operations by which the vacuum system is pumped down to high vacuum condition With optimum utilization of the mechanical and diffusion pumps and with maximum protection against damage to the diffusion oil. The circuitry is best described in connection with the overall operation of the system.

Let it be assumed that the vacuum system is completely inactivated, in which condition the components of the electrical circuitry are in the positions illustrated and in which all of the valves 16, 18, 30 are closed. The start push button switch is depressed, whereupon the electric circuit of the operate relay 162 is completed from the alternating current supply terminal 164 through the start switch contact 166 and the relay to the other supply terminal 168. Upon transfer of the contacts A, B, C, of the associated operate relay, a holding circuit is provided for the relay from the supply terminal 164 through the transferred contact B and the normally closed contact A of the inactive time delay stop relay 170, thence through the operate relay to the supply terminal 168.

Transfer of the operate relay contact B also completes the electric circuit of the mechanical vacuum pump motor 10, from the supply terminal 164 through said transferred contact B and the normally closed pump safety switch 172, through the drive motor to the other supply terminal 168.

Upon closure of the start push button switch 160, its contact 174 completes the electric circuit of the diffusion pump relay 176, .and consequent transfer of its associated contacts A, B, C provides a holding circuit for the relay from the supply terminal 164 through transferred contact B of the operate relay 162, thence through the transferred contact A of the diffusion pump relay and the normally closed stop push button switch 178, through the diffusion pump relay to the other supply terminal 168. Transfer of contact B of the diffusion pump relay completes the electric circuit of the diffusion pump heater element 180, from the supply terminal 164 through the transferred contact B of the operate relay 162 and said transferred contact B of the diffusion pump relay, through the heater element to the other supply terminal 168.

Transferred contact C of the diffusion pump relay 176 connects the resistance 182 across the capacitor 184 shunting the time delay relay 170. As explained more fully hereinafter, this relay functions upon deactivation of the system to maintain the mechanical vacuum pump operating for a period of about thirty minutes after the diffusion pump heating element has been deenergized. This delay is necessary in order to permit the hot diffusion oil to cool before the mechanical pump is stopped, thereby averting possible damage to the oil in the event of a leak in the system. Since this period of delay .is somewhat critical, it is necessary that it be consistent, and thus the resistance 182 serves to maintain the capacitor discharge prior to the start of the delay period.

The electric drive motor 24 for the respective bypass and backing valves 16, 18 carries on its output shaft 102 the rotary section 186 of a conventional wafer switch 188. The stationary section 190 of the switch is secured to the mounting bracket 62 by means of the mounting screws 192 and spacer sleeves 194 (FIG. 2). On one side of the stationary section there are mounted at circumferentially spaced positions a plurality of contacts 196, there being twelve illustrated and identified by the numerals 1 through 12 arranged in clockwise order. Some of these contacts are arranged for sliding engagement with the brush 198 carried by the rotary section, and the brush is dimensioned to make simultaneous engagement with four adjacent contacts. However, the inertia of the drive motor 24 is sufiicient to carry the brush oif of the trailing contact, when the motor is deenergized, so that the brush engages only three contacts when its rotation is stopped.

On the opposite side of the stationary section 190 of the Wafer switch there are three contacts 200 identified by the numerals 5, 8 and 11. The corresponding side of the rotary section 186 of the wafer switch carries the single radially projecting brush 202 extending from the annular conducting ring 204. The contact numbered 8 continuously engages the annular ring and the contacts numbered 5 and 11 are arranged for sliding engagement by the brush during rotation of the latter.

In similar manner, the drive motor 32 for the high vacuum flap type valve 30 also carries on its drive shaft 206 the rotary section 208 of a wafer switch. The stationary section 210 carries on one side thereof the twelve spaced contacts 212 illustrated and identified by the numerals 1 through 12 arranged in clockwise order. The rotary section carries the brush 214 which is dimensioned to make sliding engagement with six contacts 212 simultaneously, with the inertia of the motor 32 carrying the brush off of the trailing contact so that it stops in engagement with only five adjacent contacts. The opposite side of the wafer switch carries the three contacts 216 on the stationary section and the brush 218 and annular conducting ring 220 on the rotary section, in manner similar to the switch described hereinbefore.

With the switch brush 198 positioned in the valve closed position illustrated, an electric circuit is completed from the supply terminal 164 through the transferred contact A of the operate relay 162, through contact No. 7 on the fixed section 190 of the switch, through the brush 198 and contact No. 9, then through the drive motor 24 to the other supply terminal 168. The brush rotates counter-clockwise into engagement with contact Nos, 6 and 4, and stops when it leaves contact No. 7. Having rotated 90 the valve disc 84 also has rotated 90 to bring into registry the valve disc opening 100 and the bypass conduit 20. The mechanical pump 12 thus operates to evacuate the bell jar 36.

On the opposite side of the wafer switch 188 the brush 202 also has rotated 90 into engagement with contact No. 5, thus completing an electric circuit from the supply terrninal 164 through the transferred contact A of the operate relay 162, through contact No. 8, conducting ring 204, brush 202 and contact No. 5 of switch 192, thence through the heater element 222 of the thermal relay 224 to the other supply terminal 168.

The thermal relay is so arranged that when its heater element has been energized two minutes, or other desired time, the relay contact 226 is caused to close, completing the electric circuit of the cycling relay 228 from the supply terminal 164 through the transferred contact B of the operate relay 162, through the closed contact 226 of the thermal relay 224 and the normally closed contact 230 of the inactivated thermal relay 232, thence through the cycling relay to the other supply terminal 168. Transfer of the cycling relay contact A 6 provides a holding circuit which bypasses the contact 226 of thermal relay 224.

The electric circuit of the heating element 234 of the thermal relay 232 also is completed with the cycling relay 228, since they are in parallel. This thermal relay 232 is so arranged that when its heating element has been energized for thirty seconds, or other suitable time, the associated contact 230 is caused to open.

Transfer of contact B associated with the cycling relay 228 now completes the electric circuit of the valve motor 24, from the supply terminal 164 through transferred contact B of the operate relay 162, and the transferred contact B of the cycling relay, thence through contact No. 4, brush 198 and contact No. 6 of the wafer switch 188, through the drive motor 24 to the other supply terminal 168. The motor thus drives the valve disc 84 and the rotary switch section 180 (the brush 198 transferring from contact No. 4 to contact No. 1 after of rotation) to the position in which the brush interconnects the contacts Nos. 10 and 12. During this rotation, brush 202 disengages from contact No. 5 to break the circuit of the thermal relay heater 222 and open the contact 226. Having disengaged from contact No. 1, the circuit of the drive motor 24 is broken and the valve disc 84 is stopped in the position registering its opening with the backing conduit 22. The mechanical pump 12 thus pumps on the diffusion pump 26 to protect the oil, as explained hereinbefore.

Upon expiration of the aforementioned thirty seconds time delay, the thermal relay contact 230 opens, thus breaking the electric circuit of the cycling relay 228 and returning its associated contacts to the positions illustrated. The normally closed contact C completes the electric circuit from the supply terminal 164 through the normally closed contact 240 of the start push button switch 160, thence through the normally closed contact.

A of the inactive change relay 242 and the normally closed contact A of the inactive control relay 152, through the contact 244 of the manual automatic program over-ride switch for the backing valve 18, thence through the contact C of the cycling relay, contact No. 10, brush 198 and contact No. 12 of the wafer switch 188, through the electric drive motor 24 to the other supply terminal 168. The drive motor thus is energized to rotate the valve disc 180 (the brush 198 engaging contact No. 9 before disengaging contact No. 12 and engaging contact No. 6 before disengaging contact No. 9) to return the valve disc opening 100 into registry with the bypass conduit 20.

The foregoing cycle of operation repeats continually, in the event of a leak in the vacuum system, so that the mechanical vacuum pump repeatedly operates for two minutes on the bypass conduit and then for thirty seconds on the backing conduit. This latter pumping is necessary to prevent upgassing of the diffusion and consequent destruction of the diffusion oil, as previously eX- plained.

Let it now be assumed, as is normally the case in the absence of leaks, that after the first pumping on the bypass conduit 20 and the subsequent first pumping on the backing conduit 22, the vacuum system has been reduced to the pressure at which the Pirani sensing system is activated. The control relay 152 thus is energized, and its transferred contact A completes the electric circuit from the supply terminal 164 through the normally closed contact 240 of the start push button switch and the untransferred contact A of the inactive change relay 242, thence through the transferred contact A of the control relay 152 and the untransferred contact B of the inactive change relay 242, through the contact 246 of the manual automatic program override switch for the bypass valve 16, thence through contact No. 4, brush 198 and contact No. 6 of the wafer switch 188, through the drive motor 24 to the other supply terminal 168. The valve motor thus is activated for 180 rotation to bring the valve disc opening 100 to registry with the backing conduit 22.

Transfer of contact B of the control relay 152 completes the electric circuit from the supply terminal 164 through the normally closed contact 240 of the start push button switch 160 and the untransferred contact A of the inactive change relay 242, thence through said transferred contact B of the control relay 152 and the contact 248 of the manual override switch for the high vacuum flap valve 30, thence through contact No. 12, brush 214 and any one of the contact Nos. 1-4, of the flap valve wafer switch, through the flap valve motor 32 to the other supply terminal 168. The motor thus rotates 180 to open the flap valve, and the motor is deenergized when the brush 214 leaves contact No. 12 and engages contact No. 6.

With the backing valve 18 open and the flap valve 30 open, the vacuum system thus is pumped down by means of the diffusion pump 26 backed by the mechanical pump 12.

In the event of the development of a leak in the vacuum system, the Pirani element 120 decreases in resistance to the point at which the amplifier 150 ceases to conduct, as determined by the setting of the potentiometer 124. The control relay 152 thus is inactivated, whereupon the motor 32 is energized to close the flap valve 30, and the other valve motor 24 is energized to drive the valve disc opening 100 into registry with the bypass conduit 20. The alternate sequential operation of the bypass and backing valves described hereinbefore then will continue, to protect the diffusion pump.

Let it now be assumed that it is desired to open the bell jar 36, for example to introduce a sample to be evaporated. The change push button switch 250 is depressed, thus completing an electric circuit from the supply terminal 164 through the transferred contact A of the operate relay 1 62 and the closed change switch, thence through the transferred contact C of the operate relay 162 and the change relay 242 to the other supply terminal 168. Activation of the change relay transfers its associated contacts, with contacts A forming a holding circuit for the relay, through the normally closed contact 240 of the start push button switch 160. Transfer of contact C of the activated change relay 242 completes the electric circuit from the supply terminal 164 through the transferred contact B of the operate relay 162 and the transferred contact C of the change relay, thence through the contact 252 of the manual over-ride switch for the flap valve 30, through contact No. 6, brush 214 and contact No. 6 of the flap valve wafer switch, through the flap valve motor 32 to the other supply terminal 168. The flap valve thus is caused to close. Accordingly, the diffusion pump 26 is disconnected from the bell jar, but it is still connected to the mechanical pump 12 through the open backing valve 18.

An electric circuit also is completed from the supply terminal 164 through the normally closed contact 240 of the start push button switch 160 and the transferred contact A of the change relay 242, thence through contact No. 8, conducting ring 2 20 and brush 218 through contact No. 5 of the flap valve wafer switch, through the heater element 254 of the leak valve thermal relay 256 to the other supply terminal 168. This relay functions after a delay of about two seconds, which insures that the valves 18 and 30 are securely closed, to close its contact 25 8 and complete the parallel circuit of the leak valve solenoid 44. The leak valve 42 thus is opened to permit gradual admittance of atmospheric air into the bell jar. A time of about thirty seconds is suflicient to bring the bell jar to atmospheric pressure Without causing the bell jar to blow.

The specimen then is placed within the bell jar and the start push button switch 160 again is depressed. 'Momen tary opening of the normally closed contact 240 of the 8 start switch breaks the circuit of the leak valve solenoid 44 and of its delayed control relay 256, whereupon the leak valve 42 is returned to closed position. The electric circuit of the change relay 242 also is broken by opening of this normally closed contact 240, whereupon its associated contacts return to the positions illustrated. Also, since the vacuum system has been elevated to atmospheric pressure, the Pirani sensing system 40 has deenergized the control relay 152. Accordingly, the pump-down cycle of operation described hereinbefore is repeated to evacuate the bell jar.

When it is desired to inactivate the entire vacuum system, the stop push button switch 178 is depressed to break the electric circuits of the diffusion pump relay 1 76 and the diffusion pump heater 180. Return of contact C of the diffusion pump relay to the position illustrated completes the electric circuit from the direct current supply 260 through the resistance 252 and said contact C, thence through the capacitor 184 to ground. The resistance 262 is extremely large, for example 5.7 megohms, thus requiring about thirty minutes for the capacitor to charge to the firing potential of the neon tube 264. As explained hereinbefore, this time delay is necessary to permit the diffusion oil to cool before the mechanical pump is completely removed from the diffusion. Upon firing of the neon tube the time delay relay 170 is energized momentarily, and transfer of its contact A breaks the electric circuits of the operate relay 162 and mechanical pump motor 10. The electric circuits of the valve motors 24 and 32 are completed to drive the flap valve 30 to closed position and to drive the valve disc 84 to the position in which its opening is sealed from both of the bypass and backing conduits 20 and 22. Return of contact B of the now deactivated operate relay 1'62 breaks the circuit of the transformer primary 1'56 and thus inactivates the vacuum sensing circuit.

It will be apparent to those skilled in the art that various changes may be made in the details of construction described hereinbefore. For example, various other forms of sequence timing arrangements may be substituted for the motor driven wafer switches and thermal relays, although these are preferred for circuit simplicity. The foregoing and other changes and modifications may be used without departing from the spirit of this invention and the swpe of the appended claims.

*Having now described my invention and the manner in which it may be used, what I claim as new and desire to secure by letters Patent is:

1. A vacuum system comprising a diffusion pump provided with an inlet, a mechanical vacuum pump bypass conduit communicating with said inlet, a mechanical vacuum pump backing conduit communicating with the diffusion pump, a mechanical vacuum pump communicating with the bypass and backing conduits, bypass valve means in the bypass conduit, backing valve means in the backing conduit, electrically actuated drive means operatively engaging the bypass and backing valve means, valve means in the diffusion pump inlet, electrically actuated drive means operatively engaging the diffusion pump inlet valve means, and electrical timer means operatively associated with the bypass and backing valve drive actuator means for controlling operation of the latter to open and close said valve means alternately on a predetermined time sequence when the pressure in said inlet is elevated abov a predetermined value.

2. The vacuum system of claim 1 wherein the bypass and backing valve means are operated by a single electrically actuated motor and the timer means comprises rotary switch means operated by said motor, electrical time delay means having an electric circuit, the switch means being arranged in the circuits of the motor actuator and time delay means and operative with said time delay means to actuate the bypass and backing valve means on said predetermined time sequence.

3. The vacuum system of claim 1 wherein the diffusion pump includes a heater and an electric control therefor, and delay means operable by the heater control to maintain the mechanical pump operative on the difiusion pump for a predetermined time following deactivation of the diffusion pump heater.

4. The vacuum system of claim 1 including electrical vacuum sensing means communicating with the said inlet and operatively associated with the valve drive actuator means to close the bypass valve means and open the backing and diffusion pump inlet valve means when the pressure in said inlet is reduced to a predetermined value, and to close the diffusion pump inlet valve means and open the bypass valve means when the pressure in said inlet is elevated above said predetermined value.

5. The vacuum system of claim 4 wherein the sensing means includes substantially linear alternating current meter amplifier means, means interconnecting the resistance means and the meter amplifier input, and means interconnecting the meter amplifier output and an electric meter for measuring pressure.

6. A vacuum system comprising a diffusion pump provided with an inlet, a mechanical vacuum pump bypass conduit communicating with said inlet, a mechanical vacuum pump backing conduit communicating with the diffusion pump, a mechanical vacuum pump communicating with the bypass and backing conduits, bypass valve means in the bypass conduit, backing valve means in the backing conduit, electrically actuated drive means operatively engaging the bypass and backing valve means, valve means in the diffusion pump inlet, electrically actuated drive means operatively engaging the diffusion pump inlet valve means, pressure sensitive electrical resistance means communicating with said inlet, first alternating current supply means for the resistance means, means interconnecting said first supply means and resistance means to supply substantially constant current to the latter, alternating current control amplifier means, means interconnecting the resistance means and the amplifier input, second alternating current supply means, and elect-ciral vacuum control means interconnecting the amplifier output and said second alternating current supply means, the phase relationship between the first and second alternating current supply means being arranged to effect activation of the vacuum control means when the resistance means reaches a predetermined value, the vacuum control means being operatively associated with the valve drive actuator means for controlling operation of the latter to close the bypass valve means and open the backing and diffusion pump inlet valve means when the pressure in said inlet is reduced to a predetermined value, and to close the diffusion pump inlet valve means and open the bypass valve means when the pressure in said inlet is elevated above said predetermined value.

7. The vacuum system of claim 6 including electrical timer means operatively associated with the bypass and backing valve drive actuator means and operable upon deactivation of the control means to operate the drive actuator means to open and close said valve means alternatively on a predetermined time sequence when the pressure in said inlet is elevated above said predetermined value.

8. Apparatus for evacuating a system comprising: A diffusion pump having an inlet for communication with the system to be evacuated and an outlet; a bypass conduit for communication with the system to be evacuated; a backing conduit communicating with said diffusion pump outlet; a mechanical vacuum pump having an inlet communicating with said bypass and said backing conduits; bypass valve means in said bypass conduit; backing valve means in said backing conduit; inlet valve means in said diffusion pump inlet; pressure sensing means for communication with said system to be evacuated; drive means operatively engaging said bypass, backing and inlet valve means; and, control means operatively associated with said bypass, backing and inlet valve drive means and said pressure sensing means to alternately open and close said bypass and backing valve means while holding said inlet valve means closed, when the pressure in the system is above a predetermined value.

9. Apparatus according to claim 8 including additional control means operatively associated with said bypass, backing and inlet valve drive means and said pressure sensing means to hold said backing and inlet valve means open while holding said bypass valve means closed, when the pressure in the system is at or below a predetermined value.

10. Apparatus according to claim 8 wherein said diffusion pump includes a heater and an electric control therefor, and delay means operable by the heater control to maintain the mechanical pump operative on the diffusion pump for a predetermined time following deactivation of the diffusion pump heater.

References Cited by the Examiner UNITED STATES PATENTS 1,223,353 4/1917 Angell 251304 1,604,488 10/1926 Seek 25l--304 2,492,075 12/1949 Van Atta 230 2 2,920,252 1/ 1960 'Pinckaers 3 l7-149 2,936,107 5/1960 Black burn 230-2 2,943,245 6/ 1960 Ohlheiser 317-149 LAURENCE V. EFNER, Primary Examiner. 

1. A VACUUM SYSTEM COMPRISING A DIFFUSION PUMP PROVIDED WITH AN INLET, A MECHANICAL VACUUM PUMP BYPASS CONDUIT COMMUNICATING WITH SAID INLET, A MECHANICAL VACUUM PUMP BACKING CONDUIT COMMUNICATING WITH THE DIFFUSION PUMP, A MECHANICAL VACUUM PUMP COMMUNICATING WITH THE BYPASS AND BACKING CONDUITS, BYPASS VALVE MEANS IN THE BYPASS CONDUIT, BACKING VALVE MEANS IN THE BACKING CONDUIT, ELECTRICALLY ACTUATED DRIVE MEANS OPERATIVELY ENGAGING THE BYPASS AND BACKING VALVE MEANS, VALVE MEANS IN THE DIFFUSION PUMP INLET, ELECTRICALLY ACTUATED DRIVE MEANS OPERATIVELY ENGAGING THE DIFFUSION PUMP INLET VALVE MEANS, AND ELECTRICAL TIMER MEANS OPERATIVELY ASSOCIATED WITH THE BYPASS AND BACKING VALVE DRIVE ACTUATOR MEANS FOR CONTROLLING OPERATION OF THE LATTER TO OPEN AND CLOSE SAID VALVE MEANS ALTERNATELY ON A PREDETERMINED TIME SEQUENCE WHEN THE PRESSURE IN SAID INLET IS ELEVATED ABOVE A PREDETERMINED VALUE. 